Subsea solids processing apparatuses and methods

ABSTRACT

In a dual-gradient drilling apparatus, a subsea solids processing unit includes a housing, a solids processing mechanism disposed in the housing between a drilling mud inlet and a drilling mud outlet, and a flushing mechanism configured to flush the solids processing mechanism with a flushing fluid. A subsea solids processing unit may be controlled by rotating the solids processing mechanism in a forward direction and increasing a drive torque when a measured speed of the solids processing mechanism is below a desired rotation speed. The desired rotation speed may be decreased when a measured torque exceeds a selected maximum and a rotation direction of the solids processing mechanism may be reversed when the measured speed falls below a selected minimum.

BACKGROUND

1. Field of the Disclosure

Embodiments disclosed herein relate generally to apparatuses and methodsrelated to the handling and processing of drilled solids from subseawells. More particularly, embodiments disclosed herein relate toapparatuses and methods to promote proper operation of subsea pumps soas to minimize plugging process/mud return lines due to drill cuttingsentrained therein. More particularly still, embodiments disclosed hereinrelate to methods and apparatuses to ensure continuous circulation of awellbore by limiting particle size and reducing the number of largerparticles.

2. Background Art

In off-shore drilling operations, drill cuttings that are generated in awell are pumped up from the drill bit to the surface utilizing rig mudpumps on a floating drilling vessel. The drill cuttings are carried backto the surface in a fluid medium called the drilling fluid or drillingmud.

Those skilled in the art and familiar with drilling technology willappreciate that the solids and cuttings generated during a drillingoperation may span a wide spectrum of types, shapes and sizes. Theyrange from the very fine clays (e.g., 0.2-2.0 μm), to silt, sand, shale,limestone, claystone, sandstone, cement, and metal shavings generatedduring milling operations. Further, solids, such as cavings as well assloughing shale from the well, may be generated, the sizes of which mayvary anywhere from small pieces, two to five centimeters long, up totwenty-five centimeters in larger dimensions.

Along with the varying sizes of drill cuttings and solids, in certainareas in the world where the drilling operations are carried out (forexample, in the Gulf of Mexico), the clay formations may be relativelyyounger and highly reactive. The clay formations may have a tendency toform a highly sticky mass of solids known in the drilling industry as“gumbo.” These types of hydrated clay formations may be difficult todrill because they have a tendency to stick to the drill bits and havebeen known to plug pipelines and flow lines, causing a variety ofdrilling problems. Further, these clay formations are commonlyencountered in drilling operations; about 60-70% of all formationsdrilled are clays and shales.

Once the drill cuttings are generated at the drill bit, the cuttingstravel upward through an annular space, first between the open boreholebeing drilled and a drill string, and then between the installed casingand the drill string, until they reach the sea floor. From the sea floorthey may continue to the surface in the same drilling fluid mediumthrough the annular space between the drill string and a wellhead, ablowout preventer, and a marine riser. At the surface, just below therotary table, the drilling fluids along with various solids and/or thedrill cuttings, may be diverted to a shaker that separates the drillcuttings from the drilling fluid. After passing through the shakers, thedrilling fluid may circulate through further solids-processingequipment, for example, de-sanders and/or de-silters to separate sandand silt therefrom.

In drilling operations performed at large depths, the drill cuttings maybe returned to the surface in a dual-gradient drilling operation. In adual-gradient drilling operation, the returning drilling fluid does notenter the annulus of the marine riser extending from the sea floor tothe drilling rig. Instead, it is diverted away from the wellbore at thesea floor by a rotating seal assembly, so that the marine riser ismaintained full of seawater. The diverted drilling fluid along with thedrill cuttings may then enter a pumping apparatus, for example, amudlift pump. The solids laden mud entering pump chambers of the mudliftpump may be compressed and pumped up to about 265-275 liters (70-75gallons) per stroke into the mud discharge piping and may maintain aflow rate up to about 3420 liters per minute (900 gpm). Thus, the muddischarge piping runs from the sea floor to the surface, therebytransporting the drill cuttings from the sea floor to the surface. Incertain applications, the drill cuttings may be transported to afloating vessel for disposal.

Cuttings contained in drilling fluids passing through various componentsin a dual-gradient drilling operation raise significant concerns ascompared to traditional subsea operations. In conventional subseadrilling operations, the drill cuttings are carried through the annulusof the riser. Common riser sizes used today may include sizes of about54 cm (21 in) in diameter. Assuming a 6⅝ inch size drill pipe, there maybe an annular space, or radial gap between the riser and the drill pipeof about 17 cm (7 in) through which solids and drill cuttings may pass.Furthermore, larger sizes of solids are common in subsea drillingoperations from caving, sloughing, broken cement plugs, and coiledmasses of metal shavings known as “bird nests” resulting from metalmilling operations.

Available literature discusses that in order for an opening to startplugging with solids, a minimum diameter of the solid particles shouldbe at least equal to or greater than one-third a diameter of the holethrough which the solid particles pass. This is the prerequisite toinitiate bridging and to start plugging. Using this criteria, theminimum diameter of solids or the size of solids in the largestdimension that may easily pass through the present annulus may be about6 cm (2.3 in). While this is a relatively large cutting size, a riserused in a conventional drilling operation described above would be ableto pass such cuttings.

However, in a dual-gradient drilling operation, the drill cuttings andsolids must additionally pass through mudlift equipment before returningto the drilling vessel.

Mudlift pumps may have inlet and outlet lines too small to pass largercuttings and solids therethrough. For example, inlet lines of a mudliftpump may have a diameter of about 15 cm (6 in), while outlet lines mayhave a diameter of about 11.5 cm (4.5 in).

Additionally, internal chambers of the mudlift equipment may not belarge enough to accommodate some larger solids. Based on the abovecriteria, and so as not to initiate bridging and cause plugging of thereturn lines or the mudlift pump chambers, the drill cuttings would haveto be no larger than about 4 cm (1.5 in) along the largest dimension.

The above points out the importance of treating and handling solids inany type of drilling operation. Even in conventional drillingoperations, with no hindrances posed in the return drilling fluidstream, the solids may have a tendency to cause severe problems. Indual-gradient drilling, these problems may be magnified with the addedcomponents through which the drilling fluid must pass. Previously, U.S.Pat. No. 6,102,673, issued to Mott, and incorporated fully herein byreference, discloses a dual-gradient drilling concept and mentions theutilization of a solids control system to restrict the size of thesolids entering the system.

Additionally, solids returned to the surface and separated at the shaleshaker may be important from a geological standpoint. The solidsrepresent the various types of rock strata and geological formationsthat the drill bit has cut and drilled through. From the solids that areseparated at the shale shaker, small samples may be examined at regularintervals by a geologist for study and analysis. This may be especiallytrue in the case of an exploratory well where regular sample collection,tagging, study, and analysis are of paramount importance for thedetection of hydrocarbons as well as for fossil study and other tasksthat are deemed to be of geological importance. Those skilled in the artwould appreciate the importance of preserving the integrity of thematerials from the drilled formation as well as the need for itsgeological study.

Accordingly, there exists a need for a system to prevent a situationthat may cause plugging and to facilitate the entry of the solids-ladendrilling fluid into the pumping apparatus and through the return lines.Further, a system of bringing the drilled solids samples back to thesurface with as little damage and as fast as possible would be wellreceived in related industries.

SUMMARY OF THE DISCLOSURE

In one aspect, embodiments disclosed herein relate to a subsea solidsprocessing unit, including housing comprising a drilling mud inlet and adrilling mud outlet, a solids processing mechanism disposed in thehousing between the drilling mud inlet and the drilling mud outlet, anda flushing mechanism configured to wash the solids processing mechanismwith a flushing fluid. The solids processing mechanism may include afirst shaft comprising a first plurality of cutters and configured torotate about a first axis in a first direction, and a second shaftcomprising a second plurality of cutters and configured to rotate abouta second axis in a second direction, wherein the first plurality ofcutters is configured to intermesh with the second plurality of cutters.

In another aspect, embodiments disclosed herein relate to a method toretrieve drilling mud containing entrained solids from a subsea well toa surface facility including flowing the drilling mud containingentrained solids from the subsea well into an inlet of a subsea solidsprocessing unit, breaking up a portion of the entrained solids from thesubsea well exceeding a specified size with intermeshed cutters of thesubsea solids processing unit, discharging the drilling mud and thebroken-up entrained solids from the outlet of the subsea solidsprocessing unit, and pumping a solution comprising the dischargeddrilling mud and the broken-up entrained solids to the surface facility.

In another aspect, embodiments disclosed herein relate to a method tocontrol a subsea solids processing unit comprising rotating a solidsprocessing mechanism in a forward direction at a desired rotation speedwith a drive mechanism, reducing drilled solids entrained in a drillingmud with the solids processing mechanism when rotated in the forwarddirection, measuring a speed of the solids processing mechanism,measuring a drive torque of the drive mechanism with a drive torquesensor, increasing the drive torque when the measured speed of thesolids processing mechanism is below the desired rotation speed,decreasing the desired rotation speed when the measured drive torqueexceeds a selected maximum, and reversing the rotation direction of thesolids processing mechanism when the measured speed falls below aselected minimum.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

Features of the present disclosure will become more apparent from thefollowing description in conjunction with the accompanying drawings.

FIG. 1 is a perspective assembly view of a subsea solids processing unitin accordance with embodiments of the present disclosure.

FIG. 2 is a component view of a main housing of the subsea solidsprocessing unit in accordance with embodiments of the presentdisclosure.

FIG. 3A is an assembly view of a solids processing mechanism inaccordance with embodiments of the present disclosure.

FIG. 3B is an assembly view showing a meshing arrangement betweencutters of the solids processing mechanism of FIG. 3A.

FIG. 3C is an assembly view showing drill cuttings in the solidsprocessing mechanism of FIG. 3A.

FIG. 4 is a component view of a cutter in accordance with embodiments ofthe present disclosure.

FIG. 5 is a component view of a cover plate in accordance withembodiments of the present disclosure.

FIG. 6 is a schematic drawing of an elevated pressure lubrication systemin accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

As described above, mudlift pumps may have relatively small pumpchambers and fluid inlets, and therefore may require drill cuttings inthe drilling fluid to be reduced in size before entry into the pump toprevent clogging. Embodiments of the present disclosure are configuredto prevent circumstances that may cause plugging of lines and tofacilitate the entry of the drilled solids-laden mud into a mud-liftpump chamber.

Referring initially to FIG. 1, an assembly view of a subsea solidsprocessing unit (“SPU”) 100 is shown in accordance with embodiments ofthe present disclosure. Solids processing unit 100 includes acylindrical main housing 110 having a flanged fluid entry port 112 forfluid entry at the top and another flanged port 114 for fluid dischargeat the bottom. Main housing 110 further includes circular cover plates116 attached on both ends with retaining bolts 117. However, it shouldbe understood that cover plates 116 may be secured to main housing 110by any means known to a person skilled in the art, including, but notlimited to, screws, rivets, and studs.

In selecting fasteners to attach cover plates 116 to main housing 110,stresses may be determined considering all loading on the SPU 100,including pressure acting over the seal area, gasket loads, and anyadditive mechanical loads as understood by those skilled in the art. Amaximum tensile stress may be determined considering initial make-uploads of the fasteners, working conditions of the SPU 100, andhydrostatic test conditions. Stresses on the fasteners, based on aminimum cross sectional area, may not exceed the limits as described inAPI Specification 16A and understood by those skilled in the art.

Main housing 110 of SPU 100 further includes a solids processingmechanism 120 disposed therein and described in more detail below.Solids processing mechanism 120 may be powered by a drive mechanism 140attached on an end of main housing 110. Drive mechanism 140 may includea variable speed hydraulic motor driven by a variable dischargehydraulic pump and may be configured to selectively run in either aforward or a reverse direction. In alternative embodiments, electric orother motor types as would be known to one of ordinary skill may be usedfor drive mechanism 140. However, it may be advantageous to use ahydraulic motor for drive mechanism 140 as hydraulic motors may providedecreased shock loads and vibrations to the components of the SPU 100during operation.

Referring briefly to FIG. 2, a component view further detailing mainhousing 110 in accordance with embodiments of the present disclosure isshown. As shown, main housing 110 may be configured as a cylindricalshell, although one of ordinary skill will appreciate that main housing110 may also be manufactured in alternative geometries. As shown in FIG.2, main housing 110 includes multiple rows of ports (i.e. threadedholes) 115 for the attachment of nozzles 131 of a flushing mechanism(130 of FIG. 1, described in further detail below). Furthermore, inselect embodiments, main housing 110 may be constructed to maintain aworking pressure rating of about 5000 psi and may also conform to ASMESection VIII, Division 2 of the Boiler and Pressure Vessel Code as wouldbe understood by those having ordinary skill in the art.

Referring now to FIGS. 3A-3C, the various components and configurationof solids processing mechanism 120 will be described. Referringspecifically to FIG. 3A, an assembly view of solids processing mechanism120, in accordance with embodiments of the present disclosure, is shown.Solids processing mechanism 120 includes a first shaft 126 configured torotate in a first direction as shown by direction arrow A, and a secondshaft 226 configured to rotate in an opposite direction as shown bydirection arrow B. Shafts 126 and 226 each carry a set of cutters 122and 222 that may be generally co-axially positioned with respect to theshaft and are configured to rotate with shafts 126 and 226. While solidsprocessing mechanism 120 is depicted in FIG. 3A as having two shafts126, 226, solids processing mechanisms including additional shafts ofcutters should not be considered outside the scope of the presentdisclosure.

Solids processing mechanism 120 may also include a plurality of spacersor combing fingers 124 and 224 located proximally to shafts 126 and 226and spaced between the cutters 122, 222 along axes of shafts 126, 226.Although spacers 124, 224 are shown as extensions from the frame ofsolids processing mechanism 120 that extend toward shafts 126, 226, itshould be understood that shafts 126, 226 may also include spacingstructures (not shown) mounted and spaced axially thereupon betweenadjacent cutters 122, 222 and axially aligned with spacers 124, 224.Such spacing structures may, in addition to providing spacing functions,facilitate the installation of spacers 124, 224 upon shafts 126, 226 aswell as facilitate the installation of shafts 126, 226 within solidsprocessing mechanism 120.

Referring still to FIG. 3A, both shafts 126 and 226 may be supported onthe their respective ends by a bearing (not shown) and a pressure sealcartridge (not shown), either of which may be mounted in a shaft housing(not shown) as would be understood by a person having ordinary skill.Further, the shaft housings for shafts 126 and 226 may be integrallyconnected by a side frame (not shown) on both ends, thereby forming asingle-unit solids processing mechanism 120. In this manner, solidsprocessing mechanism 120 may be configured as a single unit or cartridgewhich is removable apart from main housing 110 of solids processing unit(100 of FIG. 1).

Referring now to FIG. 3B, an arrangement of cutters 122 and 222 ofsolids processing mechanism 120 in two columns generally correspondingto shafts 126 and 226 of FIG. 3A will be described. As shown, dependingon the length of shafts (126, 226 of FIG. 3A) and the widths of cutters122, 222 (and corresponding spacers 124, 224 of FIG. 3A), the number ofcutters 122, 222 on each shaft 126, 226 may vary. Therefore, as shown inFIG. 3B, cutters 122 include an array of cutters 122 extending fromfirst cutter 122 a to last cutter 122 t separated by a plurality ofaxial gaps 128, and cutters 222 include an array of cutters 232extending from first cutter 222 a to last cutter 222 t separated by aplurality of axial gaps 228.

Referring collectively to FIGS. 3A and 3B, cutters 122 and 222, andspacers 124 and 224, are shown configured along an axial length of theshafts 126 and 226 in such a way as to form an intermeshing arrangementbetween cutters 122 and 222. To achieve such an intermeshingarrangement, cutters 122, 222 and spacers 124, 224 may be configuredsuch that each cutter 122 on shaft 126 is axially aligned with a spacer224 on shaft 226 and each cutter 222 on shaft 226 is axially alignedwith a spacer 124 on shaft 126. In this manner, solids processingmechanism 120 may be configured to have a desirable clearance betweenthe cutters on opposite shafts to provide a high degree of shearingaction to the drill cuttings passing through the SPU 100.

As would be understood by one having ordinary skill in the art, axialgaps 128, 228 may have a width (in the axial direction of shafts 126,226) that is either about the same width as spacers 124, 224 or slightlywider than spacers 124, 224 by a specified clearance. The amount ofspecified clearance may be a design consideration for solids processingmechanism 120 that may selected as a function of drilling fluidviscosity, drilling fluid composition, desired cuttings size,anticipated cuttings material, anticipated operating temperature, thematerial properties of components used in the solids processingmechanism 120, the manufacturing tolerances of the components used inthe solids processing mechanism 120, any other property that may affectthe selected gap, or a combination thereof. While axial gaps 128, 228(i.e., the spaces occupied by spacers 124, 224 along axes of shafts 126,226) and cutters 122, 222 are depicted as having a constant width in thedirection along the axes of shafts 126, 226, it should be understoodthat variable widths for cutters 122, 222 and gaps 228 may be used.

Referring now to FIG. 3C, an overhead view of solids processingmechanism 120 with drilled solids 50 in position to be processed inaccordance with embodiments of the present disclosure is shown. Theintermeshing arrangement between cutters 122, 222 and spacers 124, 224of opposite shafts (126 and 226 of FIG. 3A) is shown with minimalclearances between adjacent cutters 122 and 222, as previouslydescribed. This configuration may prevent any larger drilled solids 50from passing through the solids processing mechanism 120 without beingreduced in size and may apply higher shearing forces to drilled solids50.

Referring briefly to FIG. 4, a component view of a cutter 122 (or 222)in accordance with embodiments of the present disclosure is shown. Asshown, cutter 122 may include multiple cutting edges 125circumferentially arranged about an outer perimeter of cutter 122defined by an axis of shaft 126 or 226 (of FIG. 3A) extendingtherethrough. As cutters 122 rotate on shafts 126 and 226 in oppositedirections, (i.e., as opposite facing cutting edges 125 on axiallyadjacent cutters 122 and 222 approach each other), they provide acrushing and “grinding” action on the drilled solids, thereby reducingthe solids in size. A polygonal profile 132 at the center of cutter 122prevents slippage between cutter 122 and shaft 126. Additionally, whilecutter 122 is depicted in FIG. 4 as having five cutting edges 125, oneof ordinary skill in the art will appreciate that fewer or additionalcutting edges 125 may be used without departing from the presentdisclosure. Furthermore, as shown in the intermeshing arrangement ofFIG. 3B, the radial height of cutting edges 125 and an outer diameter ofspacers 124 and 224 may be set such that as cutters 122 and 222 arerotated about shaft 126 and 226, a selected distance between the cuttingedges 125 and the outer diameter of spacers 124, 224 may be maintained.Those having ordinary skill will appreciate that by adjusting theselected distance, different sizes and geometries of processed solidsmay be achieved.

As would be understood by one having ordinary skill, it may beadvantageous for all metallic materials used in solids processing unit100, particularly internal components exposed to wellbore fluids, tomeet the requirements of NACE Standard MR-01-75 for “sour” (i.e., hightemperature and/or high sulfur) service. In addition to the metallicparts, all non-metallic parts as well as elastomeric parts may beexposed to salts, solvents, base oils, and other additives known in theart. Further, pressure-containing and controlling members, such as themain housing and cover plates, as well as the solids processingmechanism 120, may be manufactured from materials with chemicalcompositions that meet the requirements of API Specification 6A. Furtherstill, the use of dissimilar metals and exposure to corrosive internaland external environments may require cathodic protection, coating, orother practices as known to those skilled in the art or as recommendedin API Specification 17A.

Referring now to FIG. 5, in select embodiments, drive mechanism 140,attached to cover plate 116 of solids processing unit 100, may connectto an input shaft 141 of solids processing mechanism 120 (of FIGS. 1 and3A-C) extending through an opening 118 in cover plate 116. As would beunderstood by one of ordinary skill, a sealing mechanism (e.g., o-rings,packers, etc.) may be located within or around opening 118 to isolatedrive mechanism 140 on one side of cover plate 116 from solidsprocessing mechanism 120 on the other side.

As described above, drive mechanism 140 may include a hydraulic motor,an electric motor, or any other type of power generation mechanism knownto one having ordinary skill in the art. Similarly, drive mechanism 140may be connected to input shaft 141 of solids processing mechanism 120through any known mechanical means (e.g., a shaft key) known to thosehaving ordinary skill. Additionally, shafts 126 and 226 (shown in FIGS.3A-C) may be linked together by a gearing mechanism (not shown) at theend of the input shaft 141, wherein the gearing mechanism may beconfigured to allow the shafts 126, 226 to rotate in oppositedirections.

Optionally, the gears of the aforementioned gearing mechanism may becontained within an elevated pressure lubrication system such that thepressure of the lubricating oil is higher than the pressure of the drillcuttings entering the solids processing mechanism 120. Havinglubrication oil at a higher pressure allows the lubricating oil toescape to the solids processing mechanism 120 in the event of a leak,rather than allowing fluids from the solids processing mechanism 120 toescape to (and contaminate) the lubricating system.

Referring briefly to FIG. 6, an example of an elevated pressurelubrication system 300 usable with a solids processing mechanism 320 inaccordance with embodiments disclosed herein is shown schematically.Elevated pressure lubrication system 300 is shown having a firstisolation cylinder 302 and a second isolation cylinder 304. A singlepiston shaft 306 connects pistons 308, 310 contained within the firstand second isolation cylinders 302, 304. Alternatively, each isolationcylinder (302, 304) may be constructed having pistons (308, 310) onseparate shafts if the two shafts (306A, 306B) are mechanicallyconnected together. Furthermore, first isolation cylinder 302 (andtherefore first piston 308) comprises a bore larger than secondisolation cylinder 304 (and therefore second piston 310). An oilreservoir 312 connects an output line 314 of first isolation cylinder302 to an input 316 of second isolation cylinder 304. Thus, a pressureapplied to an input 322 of isolation cylinder 302 will be magnified bythe ratio of cross-sectional area of pistons 308 and 310 at an output318 of second isolation cylinder 304.

As such, a pressurized mud (PMUD) from an input 330 or an output 332 ofa solids processing mechanism 320 may pressurize oil in input line 322of first isolation cylinder 302 (in select embodiments, through apressure isolator 334) to exert a force on piston 308. Force upon piston308 displaces piston 308 to the right which, in turn, also displacessecond piston 310 (coupled by shaft 306) to the right by the sameamount. Thus, as piston 310 is larger than piston 308 by a known ratio,the pressure of the lubricating oil P_(OIL) contained in lubricating oilsupply 336 and thrust by larger, second piston 310 may be increased overthe mud pressure P_(MUD) by a factor equal to the ratio of thedifferential cro-sectional areas of the first 308 and the second 310pistons. Thus

$\begin{matrix}{{P_{OIL} = {P_{MUD} \times \frac{A_{310}}{A_{308}}}};} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$where A₃₁₀ is the cross-sectional area of second piston 310 and A₃₀₈ isthe cross-sectional area of first piston 308.

Alternatively, in select embodiments, rather than including a gearingmechanism to link shafts 126, 226 as described above, two drivemechanisms, one for each shaft, may be attached and configured toindependently rotate shafts 126, 226. While it may be desirable torotate shafts 126, 226 in opposite directions and at equal speeds, theremay be circumstances where it may be beneficial to rotate shafts 126,226 in the same direction or at differing speeds.

Additionally, as shown in FIG. 1, solids processing unit 100 may furtherinclude a flushing mechanism 130 attached to an outer surface of mainhousing 110 configured to deliver a fluid to clean the cutters of solidsprocessing mechanism 120. As shown in FIG. 1, flushing mechanism 130 mayinclude rows of nozzles configured to spray fluid onto solids processingmechanism 120.

As mentioned earlier, the generated drilled cuttings may contain up to60-70% of clay or shale type formations, which may exhibit a tendency tostick to metal and gradually build up. In solids processing mechanism120, cuttings may build up on the cutters 122, 222 and the areas formedby the curvature of cutting edges of cutters 122, 222. This may lead topossible plugging or blockage of solids processing mechanism 120. Assolids processing unit 100 may be installed in deep water on the seafloor, if solids processing mechanism 120 plugs, expensive retrieval andreplacement operations may be required.

In order to prevent a plugged solids processing mechanism 120, flushingmechanism 130 may provide fluid flow at high velocity and/or pressurethrough multiple nozzles onto solids processing mechanism 120 duringoperation of solids processing unit 100. In select embodiments, flushingmechanism 130 may continuously clean or “flush” material off the cuttersto prevent material build-up. The fluid stream exiting the nozzles offlushing mechanism 130 may have sufficient energy to flush or cleancutters 122 (in particular, edges 125) of solids processing mechanism120 and remove stuck cuttings.

In select embodiments, the fluid used by flushing mechanism 130 to cleancutters 122, 222 may include “clean” drilling fluid (i.e., drillingfluid without entrained solids) supplied from the surface by thedrilling rig or another vessel. In select embodiments, the fluid may bepumped down using a high head centrifugal pump or a triplex piston mudpump to attain sufficient cleaning velocities.

In select embodiments, the nozzles may be replaceable and may vary indiameter from 6 mm (¼ in) up to 26 cm (1 in). Sizing of the nozzles maybe based on optimal or required flow rate, pump capabilities, drillcuttings material, and other requirements understood by those skilled inthe art. Further, ports that are equivalent to nozzles may be used forflushing and may comprise a concentric hole drilled through a hex-headedbolt. Each bolt may be fitted directly to the threaded port 115 (of FIG.2) so that flow through the bolt may be directed to the solidsprocessing mechanism 120. Further, an o-ring or other seal mechanism maybe used to seal between the surfaces of the head of the bolt and mainhousing 110 of solids processing unit 100.

In select embodiments, nozzle size may be selected based on at least twocriteria. First, an optimal flow rate may be computed based on the levelof flushing and cleaning desired, which may be calculated on the basisof maximizing the hydraulic horsepower available for cleaning the solidsprocessing mechanism 120. Second, the size of the nozzles may be basedon the pressure head developed by the hydrostatic column of mud abovethe nozzles combined with the head imparted by the high head centrifugalpump, such that the fluid expansion through the nozzles will have aminimal impact on the open formation through the annulus. Anotherpossibility for ensuring that the pressure is not transmitted to theformation is through the use of a suitable fixed or variable choke inthe line carrying the clean mud for flushing.

Referring to FIGS. 1-5, operation of solids processing unit 100 may bedescribed as follows in accordance with select embodiments of thepresent disclosure. During the operation of the SPU 100, drilling mudcontaining drill cuttings (i.e., entrained solids) flows from a downholelocation toward the mud-lift pumping equipment as described above.Solids processing unit 100 may be positioned just before the mud liftpumps to reduce the drill cuttings size and prevent plugging of the mudlift pumping equipment. The drill cuttings enter the SPU 100 throughfluid entry port 112 on top of main housing 110. As the drill cuttingspass through SPU 100, cutters 122 and 222 of solids processing mechanism120 rotate in the direction of arrows A and B, shown in FIGS. 3A and 3B,providing shearing, crushing, and grinding actions to reduce the size ofthe drill cuttings.

As the cuttings are crushed and ground into smaller pieces, they may bereduced to a small enough size to pass through solids processingmechanism 120 and exit SPU 100 through fluid discharge port 114 on thebottom of main housing 110. Other drill cuttings may be small enough topass through SPU 100 without being reduced and may continue to the rigsurface, providing unmodified geological samples for analysis.

For example, in one application, solids processing unit 100 may beconfigured to reduce the size of all drilled solids larger than 3.8 cm(1.5 in) in diameter to an average diameter of 1.2 cm (0.5 in), but notaffect any drilled solids smaller than 3.8 cm (1.5 in). Because thesolids processing mechanism 120 would be configured to crush pieceslarger than 3.8 cm (1.5 in) into much smaller pieces, a technicianevaluating samples passing through the SPU 100 may deduce that largerparticles passing through the SPU 100 (i.e., those between 12 cm (0.5in) and 3.8 cm (1.5 in) in size) were not reduced by the solidsprocessing mechanism 120. Thus, through selective sampling, a surfacetechnician may be able to differentiate (at the surface) particlesbypassing solids processing mechanism 120 from those reduced by solidsprocessing mechanism 120.

In select embodiments, a control system used with the solids processingunit 100 may vary the cutting speed and torque in order to “cut through”certain materials. For example, a control system may incorporate a bladespeed sensor to measure the rotational speed of blades 122, 222 onshafts 126, 226 and a drive torque sensor to measure an output torquesupplied to solids processing mechanism 120 by drive mechanism 140. Byusing the blade speed sensor, solids processing unit 100 may becontrolled such that blades 122, 222 of solids processing mechanism 120are rotated at a selected maximum forward speed (i.e., a maximum forwardspeed set point) and that a torque output from drive mechanism 140 maybe controlled (i.e., increased or decreased) to maintain the selectedmaximum forward speed.

In the event that drive mechanism 140 cannot provide enough torque tomaintain the selected maximum forward speed, then the forward speed maybe reduced. In the event that the torque output of the drive mechanismis insufficient to maintain a selected minimum forward speed (i.e., aminimum forward speed set point), the rotation of the blades 122, 222and solids processing mechanism 120 may be reversed (at a selectedreverse speed) by the controller to clean out the solids processingmechanism 120.

Similarly, the torque output of the drive mechanism may be controlled,(i.e., increased or decreased) to maintain the selected reverse speed.After a specified amount of rotation, the control system may stop thereverse rotation and forwardly rotate the solids processing mechanism120 again at the selected maximum forward speed. In the alternative, thecontrol system may stop the reverse rotation after the solids processingmechanism is reversed for a specified amount time at a drive torquelower than a selected reverse torque limit. Furthermore, if the drivemechanism exceeds a maximum torque in either the forward direction orthe reverse direction (indicating a potential jam or malfunction in thesolids processing mechanism 120), the control system may be configuredto shut down the solids processing unit 100 until the solids processingmechanism can be inspected, manually cleaned, or replaced so that othercomponents of solids processing unit 100 may not be damaged.

As such, the SPU 100 may be operated both in a manual mode and in anautomatic mode. Increasing the speed (RPM) of solids processing unit 100may be accomplished by sending a signal to a hydraulic drive pump toincrease the fluid flow rate to the motor driving the solids processingmechanism 120. The RPMs at which shafts 126, 226 of the solidsprocessing mechanism 120 rotate may be sensed by a pulse sensortransmitting the speed to the operator as would be understood by thoseof ordinary skill in the art.

Solids processing unit 100 may also be programmed to run in an automaticmode.

In select embodiments, control of the solids processing unit 100 may bebased on a set of measured pressures termed “high-side” and “low-side”pressures. The pressures may be measured at similar locations forconsistency, such as the fluid entry port, fluid discharge port, or anyother appropriate location as would be understood by a person skilled inthe art. For example, detection of a high-side pressure at the fluiddischarge flange may be an indication that the SPU 100 may be rotatingin the forward direction, where it may “grip” the drilled solids andprocess them (i.e., reduce in size). Similarly, presence of a low-sidepressure at the discharge port may indicate that the SPU 100 is rotatingin the reverse direction, where it may dislodge solids that stick orbind up in the SPU 100.

The sequence of operation of the SPU 100 may be controlled by analgorithm based on several set pressures, or “set points,” each of whichmay be set by an operator. Further, whenever measured high-side orlow-side pressures reach particular set points, certain actions may becarried out by the SPU 100, including stopping, reversing, and rotatingat selected RPMs. The set points and their operating modes are explainedfurther below.

A Maximum Forward Pressure (“MFP”) set point may correspond to a highestallowable developed pressure when the SPU 100 runs in the forwarddirection. In certain embodiments, this set point allows the SPU 100 torun in the forward direction at about 20 RPM. If the SPU 100 isexperiencing any load due to excess drill cuttings or any difficulty inprocessing solids because of their size, the high-side pressure mayincrease. When the high-side pressure exceeds the Maximum ForwardPressure, a signal may be sent to stop and/or reverse SPU 100 rotation.

A Reverse Number Of Teeth (“RNOT”) set point may be a fraction of a fullturn that the SPU 100 will reverse to dislodge stuck solids, the amountof rotation being determined by a specified number of teeth in the drivegear. Therefore, an entry of “24” as the Reverse Number Of Teethset-point for a drive gear having twenty-four teeth would cause theshafts of the SPU 100 to reverse one complete revolution. As describedabove, the reverse mechanism incorporated in the control system may actas a safety against large pieces of solids that may be difficult toprocess in one pass and may possibly overload the system.

A “Maximum Reverse Pressure” (“MRP”) set point may be equal to thelimiting pressure set point when the SPU 100 runs in the reversedirection. If the low-side pressure increases beyond the Maximum ReversePressure set point while reversing, the SPU 100 may be programmed tostop rotating in the reverse direction and/or to start rotating forwardagain. One purpose of the Maximum Reverse Pressure set point may be toprevent overloading of the SPU 100 in the reverse direction and toprevent the SPU 100 from getting stuck while attempting to dislodge thesolids overloading the SPU 100.

A “Maximum Pressure at Low Speed” (“MAXPLS”) set point may also beincluded in an SPU 100 control sequence. As described above, after theSPU 100 reverses a selected number of teeth, the SPU 100 maysubsequently rotate forward at a lower speed (e.g., about 8 RPM inselect embodiments) and with a higher amount of torque. If, whilerotating in the forward direction at the lower speed, the SPU 100high-side pressure increases above a Maximum Pressure at Low Speed setpoint, the SPU 100 may stop and rotate in reverse. This control step maybe incorporated to protect the components as well as the system fromoverloading. After changing direction and rotating in reverse, in selectembodiments the SPU 100 may repeat the sequence of steps (RNOT, MRP,etc.) described above.

Furthermore, a “Minimum Pressure at Low Speed” (“MINPLS”) set pointpressure may also be implemented in an SPU 100 control scheme. If afterrunning at low speed for a select amount of time, the high-side pressurehas decreased to a value that is below the MINPLS, the SPU 100 maychange from the lower speed back to the higher speed (e.g., 20 RPM) andcontinue rotating in the forward direction. Furthermore, the controlsystem set point would likewise change from MAXPLS back to MFP.

As such, a method to retrieve drilling mud containing entrained solidsfrom a subsea well may include breaking up (i.e., reducing the size of)a portion of the subsea solids at or near the sea floor. From there, thereduced solids may either be disposed of at the sea floor or may bedischarged (along with the drilling mud and remaining entrained solids)to a surface facility. The retrieval may also include breaking up theportion of the entrained solids with intermeshed cutters of a solidsprocessing unit. The method to retrieve may also include bypassingentrained solids that are smaller than a specified size around thesolids processing unit to an outlet of the solids processing unit. Thedrilling mud and entrained solids, whether broken-up or bypassed, may bedischarged from the outlet and pumped to the surface facility. As usedherein, “surface facility” may be any storage, processing, disposal,measurement, recycling, or other facility that is located “above” thelocation of the subsea well. Therefore, a surface facility may include,but should not be limited to, a floating vessel, a tethered or anchoredplatform, or a land-based facility.

Advantageously, embodiments disclosed herein may prevent plugging orstoppage of return lines and/or mud lift pump chambers. Because mud ispumped back to the surface using a positive displacement diaphragm pumpin dual-gradient drilling operations, it may be beneficial to ensurethat there is no plugging of either the lines or the pump chamber belowthe diaphragm with large solids. Primarily because, in the event ofplugging of the line or the pump, it becomes difficult to revert to asingle gradient in the well without loss of the well. Therefore,embodiments disclosed herein may greatly reduce costs and drilling timeby preventing downhole failures.

Also, those skilled in the art will appreciate that it may beundesirable to allow large pieces of solids to accumulate in the wellbore where they may pose a potential threat by way of increased drillingtorque, bit drag, tight-pull, hole pack-off, and various other drillingproblems that may result. Maintaining large amounts of solids in theannulus may also increase the effective density of the fluid, therebyincreasing the effective bottom hole pressure on the formation and thusincreasing the risk of fracture. Advantageously, embodiments disclosedherein may help ensure that large solids are not left in the well orallowed to agglomerate and grow. Rather, a solids processing unit inaccordance with embodiments disclosed herein (e.g., 100) may beconfigured to process solids in the mud flow stream.

Further, embodiments disclosed herein may also provide a moreenvironment-friendly solids processing operation. In some dual-gradientdrilling applications, drilled solids would otherwise be separated anddumped on the sea floor. Solids processing units in accordance withembodiments disclosed herein are particularly advantageous in view ofstringent environmental regulations currently in effect in several partsof the world (e.g., the North Sea and the Gulf of Mexico), thuseliminating or substantially reducing the need to deposit the cuttingson the sea floor, especially when oil-based drilling fluids are used.Instead, the drilled solids may be recovered to the surface,particularly in sizes acceptable for geological study.

Still further, embodiments disclosed herein may allow better geologicalsamples of drill cuttings to be obtained by geologists at the surface.With very small solids, the geological information contained within thecuttings may be compromised. Further, the smaller solids maybeneficially affect mud properties such as density, plastic viscosity,and the gel strength, all of which may improve substantially as the sizeof entrained solids in the returning mud decreases.

Select embodiments disclosed herein work by macroscopically reducing thesize of only the larger particles to no greater than a specifieddimension. Particles that are smaller than the specified dimension maypass through without being processed. Thus, the integrity of thecuttings is maintained and their sized kept large enough for geologicalstudy and analysis without affecting the properties of the mud, but notso large as to clog or impede the fluid flow through the subsea pumpingequipment.

While the present disclosure has been described with respect to alimited number of embodiments, those skilled in the art, having benefitof this disclosure, will appreciate that other embodiments may bedevised which do not depart from the scope of the disclosure asdescribed herein. Accordingly, the scope of the disclosure should belimited only by the attached claims.

1. A subsea solids processing unit for comminuting cuttings in drillingmud being returned from a well being drilled, comprising: a housinghaving a drilling mud inlet and a drilling mud outlet, the housingdefining a pressure containing vessel capable of being operated in asubsea environment, the drilling mud inlet adapted to be connected to asubsea drilling mud return flow line; a solids processing mechanismdisposed in the housing between the drilling mud inlet and the drillingmud outlet, the solids processing mechanism comprising: a first shaftcomprising a first plurality of cutters and configured to rotate about afirst axis in a first direction; a second shaft comprising a secondplurality of cutters and configured to rotate about a second axis in asecond direction, wherein the first plurality of cutters are configuredto intermesh with the second plurality of cutters; a flushing mechanismconfigured to flush the solids processing mechanism with a flushingfluid; a lubricant reservoir containing a lubricant for supplyinglubricant to portions of the unit; a pressure intensifier connected withthe lubricant reservoir for applying a pressure to the lubricant beingsupplied to the unit that is responsive to and higher than drilling mudpressure at the drilling mud inlet or drilling mud outlet.
 2. The subseasolids processing unit of claim 1, wherein the flushing mechanismcomprises at least one nozzle disposed on the housing and flushes thefirst and second pluralities of cutters of the solids processingmechanism with the flushing fluid; and the flushing mechanism is adaptedto be connected to a drilling mud inflow line for flushing the cutterswith drilling mud bring pumped into the well.
 3. The subsea solidsprocessing unit of claim 1, wherein the flushing mechanism comprises afirst row of nozzles disposed adjacent to the first and the secondpluralities of cutters.
 4. The subsea solids processing unit of claim 1,further comprising: a first plurality of spacers positioned between thefirst plurality of cutters along the first axis; and a second pluralityof spacers positioned between the second plurality of cutters along thesecond axis.
 5. The subsea solids processing unit of claim 1, furthercomprising: a controller having an automatic mode wherein the controllercontrols rotation of the first and second shafts based on a pressuredifferential between drilling mud pressure at the drilling mud inlet andthe drilling mud outlet.
 6. The subsea solids processing unit of claim5, wherein the controller is configured such that while in the automaticmode, a pressure differential exceeds a selected maximum, the controllercauses the first and second shafts to reverse rotation.
 7. The subseasolids processing unit of claim 6, wherein after the controller isconfigured such that after the controller has reversed rotation inresponse to a pressure differential in excess of the selected maximum,the controller causes forward rotation but at a lower speed and highertorque than employed when the pressure differential exceeded theselected maximum.
 8. The subsea solids processing unit of claim 1,further comprising: a controller that automatically varies the speed ofrotation of the cutters in response to torque applied to the first andsecond shafts.
 9. The processing unit of claim 1, further comprising: acontroller that automatically reverses rotation of the first and secondshafts in the event a minimum forward speed is not attainable.
 10. Theprocessing unit of claim 9, wherein the controller automatically returnsto forward rotation after a selected period of reverse rotation.
 11. Asubsea solids processing unit for comminuting cuttings in drilling mudbeing returned from a well being drilled, comprising: a housing having adrilling mud inlet and a drilling mud outlet, the housing defining apressure containing vessel capable of being operated in a subseaenvironment, the drilling mud inlet adapted to be connected to a subseadrilling mud return flow line; a solids processing mechanism disposed inthe housing between the drilling mud inlet and the drilling mud outlet,the solids processing mechanism comprising: a first shaft comprising afirst plurality of cutters and configured to rotate about a first axisin a first direction; a second shaft comprising a second plurality ofcutters and configured to rotate about a second axis in a seconddirection, wherein the first plurality of cutters are configured tointermesh with the second plurality of cutters; a controller that sensesrotational conditions of the first and second shafts and varies rotationof the first and the second shafts in response; and wherein thecontroller has an automatic mode wherein the controller controlsrotation of the first and second shafts based on a pressure differentialbetween drilling mud pressure at the drilling mud inlet and the drillingmud outlet.
 12. The subsea solids processing unit of claim 11, whereinthe controller has a speed sensor that senses the rotational speeds ofthe first and shafts.
 13. The subsea solids processing unit of claim 11,wherein the controller is configured such that while in the automaticmode, a pressure differential exceeds a selected maximum, the controllercauses the first and second shafts to reverse rotation.
 14. The subseasolids processing unit of claim 13, wherein after the controller isconfigured such that after the controller has reversed rotation inresponse to a pressure differential in excess of the selected maximum,the controller causes forward rotation but at a lower speed and highertorque than employed when the pressure differential exceeded theselected maximum.
 15. A subsea solids processing unit for comminutingcuttings in drilling mud being returned from a well being drilled,comprising: a housing having first and second drilling mud ports, thehousing defining a pressure containing vessel capable of being operatedin a subsea environment, the first drilling mud port adapted to beconnected to a subsea drilling mud return flow line for receivingdrilling mud flowing from the well, and the second drilling mud portadapted to be connected to a subsea pumping unit for delivering thedrilling mud flowing from the housing to the subsea pumping unit; asolids processing mechanism disposed in the housing between the firstand second drilling mud ports, the solids processing mechanismcomprising: a first shaft comprising a first plurality of cutters andconfigured to rotate about a first axis in a first direction; a secondshaft comprising a second plurality of cutters and configured to rotateabout a second axis in a second direction, wherein the first pluralityof cutters are configured to intermesh with the second plurality ofcutters; a drive motor; a gear mechanism operatively coupled between thefirst and second shafts and the drive motor for causing rotation of thefirst and second shafts; a lubricant reservoir containing a lubricantfor supplying lubricant to the gear mechanism; a pressure intensifierhaving at least one piston located within a cylinder; an input linecommunicating fluid pressure of the drilling mud in one of the first andsecond drilling mud ports to the pressure intensifier, which produces afluid output pressure proportional to but greater than the fluidpressure of the drilling mud in one of the first and second drilling mudports; and an output line leading from the pressure intensifier to thelubricant reservoir connected with the lubricant reservoir for applyingthe output fluid pressure to the lubricant being supplied to the unit.16. The subsea solids processing unit of claim 15, further comprising anisolator in the input line that isolates drilling mud from lubricantcontained in the lubricant reservoir.
 17. The subsea solids processingunit of claim 15, wherein said at least one piston comprises twopistons, one having a diameter larger than the other.